Optical communication network nodes, such as reconfigurable optical add/drop multiplexers, ROADM, typically perform optical signal power leveling on a per signal basis in order to maintain the power level of each optical signal within a preset range. Each node comprises an optical channel monitor, OCM, arranged to measure the optical signal power of each signal, and an optical attenuator arranged to apply a respective attenuation to each optical signal; in a ROADM the attenuation is typically applied by a wavelength selective switch, WSS, used to switch the optical signal with a per-wavelength granularity.
There is a given accuracy in the OCM optical signal power reading, that is not zero, (and in the WSS setting as well). This error in the optical signal power measurement of a signal can cause an adjustment of the WSS attenuation when the optical signal power is actually stable; the node interprets the reading error as a real difference between the actual optical signal power and a target optical signal power, and adjusts the attenuation accordingly. With an optical signal having a stable optical signal power what happens is that the reading error from the OCM is considered to be a “real” power error to be followed, which results in an attenuation variation being applied to the WSS, creating a real offset with respect to a target power of the optical signal. The resulting change in the optical signal power, delta power, caused by the OCM measurement error, is negligible at the output of an individual node. But where there is a chain of tens of nodes the delta power is propagated to each of the nodes in the chain, with each node adding its own delta power, and since all the nodes will react independently to compensate the “cumulated” delta power oscillations in the optical signal power of each optical signal develop, which at the end of the chain can be as high as several dB, depending on the number of nodes. In a long chain of nodes this mechanism can create power oscillations, at an optical signal level, while the total power is not affected.
U.S. Pat. No. 8,135,280 discloses power stability control in wavelength division multiplexed, WDM, networks when an optical signal power needs to be adjusted and an analogous issue of power oscillation is present due to the independent response of all the nodes in a chain. However, this only solves the issue of power oscillations caused by add/drop of channels and does not prevent power oscillations occurring in a steady state condition of a network.